The present invention relates to a method of coupling a magnetic pole position detecting device with a brushless dc motor and a coupling structure therefor.
In a brushless motor, since it is necessary to electronically switch the exciting current phase according to rotor angular position during operation, it is particularly essential to detect the rotor magnetic pole angular position as precisely as possible with a magnetic pole position detecting device. When the pole position cannot be detected precisely, there arise various problems in that unavailable components of the ting current increase, motor efficiency (output), decreases; abnormal heat is generated, and motor speed increases abnormally (run-away operation) at the worst. Therefore, it is particularly important to couple the magnetic pole position detecting device with the dc motor at a predetermined angular position about a motor axis.
The above-mentioned magnetic pole position detecting device coupled to the brushless dc motor is an electronic device such as encoder or resolver, which is usually assembled accurately by manually adjusting the angular position of the detecting device relative to the dc motor. Therefore, once trouble occurs, in most cases the detecting device is replaced with a new one together with the dc motor, thus resulting in loss in labor and cost. This is because once only the magnetic pole position detecting device is removed from the dc motor for replacement, it is necessary to align both the angular positions again and protect the detecting device from dust, with the result that it is impossible to repair the detecting device in a short time.
A typical prior art method of coupling a magnetic pole detecting device with a dc motor will be explained with reference to FIGS. 1 to 4. In FIG. 1, a marking-off line 5 is scribed on an end surface of a casing 1 of a brushless dc motor, and a key 3 is pressure-fitted into a key groove 4 formed in a motor output shaft 2 as shown in FIG. 2 so that a predetermined angular position of the motor output shaft 2 can be set on the basis of the scribed marking-off line 5. In the same way, another marking-off line 14 is scribed on an end surface of a magnetic pole position detecting device 6 and another key 10 is pressure-fitted into another key groove 9 formed in a detecting device input shaft 8 as shown in FIG. 3. Further, a shaft coupling member 7 is formed with a central through hole 11 and a key groove 12 as shown in FIG. 4. Therefore, the detecting device 6 is coupled to the dc motor via the shaft coupling member 7 by matching each key 3 or 10 with each marking-off line 5 or 14 and fitting each key 3 or 10 into the key groove 12 formed in the shaft coupling member 7.
However, in the prior-art method as described above; since the shaft coupling member 7 is used, there exists a problem in that the axial length is large and therefore the rigidity between the motor shaft and the detecting device shaft is low. In addition, since the two shafts are coupled with each other at two different positions via two different keys play (looseness) is inevitably produced and additionally the forming of the key groove results in a high cost.